Gas transmission pipelines are widely used to distribute natural gas from producing gas fields to local pipeline taps, from thence across the country to local distribution stations, and from there to power utilities or individual consumers. The flow rates and pressures in such pipelines may vary over a wide range. To transport the enormous quantities of gas required, the diameter of gas mains may often reach several feet in diameter. The pipelines are generally constructed of carbon steel, as this material is relatively inexpensive, strong, and joints can be fabricated easily, for example by butt welding.
The lifetime of gas and other steel pipelines is limited, however, due to both external and internal corrosion and other factors. Moreover, there are many occasions where prior connections must be sealed off, or pipeline damage caused by earth movement, construction mishaps, and the like, must be corrected. In many such instances, pipe relining has proven to be an acceptable repair method, avoiding wholesale replacement of the pipeline or pipeline section. In cure-in-place pipeline relining, a sleeve of fiber reinforced, curable polymer is inserted along the relevant length of pipe, expanded against the pipe walls, and cured, producing a fiber-reinforced polymer composite inner lining. In slip relining, a prefabricated lining is pulled and/or pushed through the pipeline.
In order to reline a pipeline successfully, the pipeline must be cleared of debris and cleaned, and any protrusions removed. Protrusions such as service connections and plugs, etc., may be removed by robotic precision cutting and grinding tools. Steel, cast, and malleable iron nipples as large as 4 inch diameter may be removed with tolerances acceptable for relining operations. Milling cutters or "core bits" cannot be used for such removal operations because the milling cutters or bits are necessarily of a smaller diameter than the pipeline, and thus a small stub is left behind. This stub must generally be ground away. Precision grinders of the type useful in pipelines are expensive and slow in operation. The time which a pipeline is removed from service is generally critical, as customers may be without gas during this period.
Welds are more problematic. Whereas taps and plugs are generally small and restricted to a small area, allowing a precision grinding machine to be remotely positioned at the required position, welds generally occur over the entire inner circumference of the pipeline. Moreover, as most pipeline welding operations are of necessity done in the field with the pipe substantially horizontal, the amount of weld protruding from the top of the pipe may not be the same as that protruding from the sides or bottom. For example, in the relining of the Union Turnpike (Queens, N.Y.) gas main, excess welds 3/8" thick and 3/4" deep covering 50% of the pipe circumference was encountered in a 10" diameter main. To complicate matters further, the weld metal may be considerably harder than the relatively soft materials of plugs and nipples, and may be irregular as well. Robotic precision grinding tools are ill-suited for use in removing weld. For example, precision grinding tools are capable of removing excess weld as described above at the rate of about 3 to 6 joints per eight hour shift. While this rate is intolerably slow, acceptable alternatives have not existed heretofore.
In U.S. Pat. No. 4,460,920 is disclosed an articulatable, stepping manipulator useful in straight and curved pipes both horizontal and vertical, for rewelding, grinding and other operations, particularly for nuclear power plants. In the '920 device, two counterstepping bodies each have a plurality of clamping bodies to bear against the pipe. With the clamps on one body released, pneumatic or hydraulic pistons extend or retract that body from the first. Upon clamping of the moved body against the pipe walls, the other body may be unclamped and follow the first, in "inch worm" fashion. A single grinding wheel is positioned on a bearing such that the plane of the grinding wheel forms an acute approach angle tangent to the tube wall at the contact point of the grinding wheel.
The device of '920 requires an electrically driven grinding wheel, generally prohibited in much underground work, and grinds only a small portion of protruding weld at a given time. Moreover, as the grinding wheel wears, the unit must be removed, and/or the grinding wheel repositioned or replaced, otherwise the angle of attack may vary considerably. For the same reason, the '920 device can be used effectively only for a single pipe size.
In U.S. Pat. No. 5,233,791 is disclosed a tool body similar to a reamer which cuts and/or grinds interior of a pipe. However, such a tool is of necessity somewhat smaller than the pipe bore and thus cannot grind welds, nipples, etc., flush but instead must leave a slight protrusion. The driving mechanism and construction are not suitable for use in long pipelines where runs of several hundred feet may occur without machine access. U.S. Pat. No. 5,175,964 discloses an apparatus for similar service, but on which is mounted an angularly disposed grinding wheel similar to that of the '920 patent. Like the '791 patent, the '964 device is designed for use in remedying defects in relatively short lengths of non-welded pipe, i.e. oil field drill pipe and casing prior to installation, and is not suited to long runs of pipe.
U.S. Pat. No. 4,084,484 appears to disclose a welding flash removal tool used to remove weld flash immediately following resistance butt welding. The device employs rotating cutters cantilevered from a rod and mounted to a movable appliance. In use, the cutters are urged outward against the pipe by spring pressure against a hydraulic piston. The amount of working fluid within the retaining side of the piston is lowered at each revolution by the opening of a slide valve bearing against a fixed cam. Thus, the rate of feed is constant from cut to cut along the length of the pipe, although for any given cut, the feed rate varies inversely with the distance from the center of the pipe. With its invariant cutting rate, the amount of time spent in grinding a minor amount of flash is the same as for heavy flash. Moreover, the device is quite complex mechanically with numerous sliding and rotating hydraulic valves which require complex sealing arrangements. Cantilevering from a boom is also problematic, particularly in pipes of narrow cross-section. A working member cantilevered to a boom is also illustrated for use in sewer tubes (generally non-metallic) in U.S. Pat. No. 5,207,031.
U.S. Pat. No. 5,054,976 discloses an inside processing apparatus stated as useful for grinding and cleaning operations inside pipes. The apparatus consists of a single grinding wheel mounted off-center to the pipeline axis and driven by two sets of planetary gears such that rotation of the respective gear sets can position the grinding wheel at different positions around the inner circumference of the pipe. The apparatus requires numerous precision machined parts, and is capable of grinding completely through a pipeline if not monitored carefully. The device is particularly suitable for non-metallic sewer pipe relining operations where grooves to contain sealing rings are to be machined. Replaceable heads can be attached to facilitate grinding, cleaning, cutting tree roots, and the like.
None of the devices of the related art are suited for the rapid removal of weld from metal pipelines. It would be desirable to provide an apparatus suitable for the removal of welds from the inside of pipelines which is capable of high throughput. It would be further desirable to provide an apparatus which is robust yet simple construction, and adaptable to numerous pipeline diameters. It would yet be further desirable to provide an apparatus which not only has the capability of rapid weld removal but further can be used to cut off nipples, plugs and other protrusions as well as being useful for cleaning, descaling, and other operations associated with pipeline refurbishing and/or relining, or similar operations in other tubular structures.